Like it so far, hard to tell about battery performance since I've been playing with it all morning. The only thing I don't like is the lack of folder colors, I prefer gray or black and there is only 5 color options all of which I don't really like.

This update has had no issues hands down faster better battery smoother animation and overall a great update I would just take the plunge you won't regret it. T-Mobile isn't like the "other" guys they did a ton of testing before releasing it

I've only had it for an hour now. Apps load faster, MMS messages send faster, linked up to my Dodge Durango via Bluetooth in 2 seconds, where before it would take 15-30 or a need to turn off/on Bluetooth and try again. Even typing on my SwiftKey keyboard seems faster. Sometimes there was a delay.

Screen seems brighter and phone is warmer then usual, but that could be because of everything installing/updating post update.

Update:
Can't get the phone to be at a "resting" temperature, which normally for me was between 75-85°F. Anything above that and I knew someone was eating my battery in the background. It's staying in the 92-96°F since updating to Marshmallow.

Turned off WiFi calling, Voice over LTE, location, and even auto sync. Cleared cache from the bootloader menu already.

Got it down to 80°F by putting it in the freezer for 2 minutes, but it's back to 96° now.